This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons folder. You should end up with a directory structure similar to below:
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
optimusmikee, Tomekiac78, Nephilim, nantawtgolf2499, lizzymtz2, desinteresado124, kitsbaby, [KR] Seawhite, 1228256275, Mario44422 ヽ(⩾Ѡ⩽)ﾉ, HandsomePalpi194, Burger, cobra, Porter Justice, laureanotymczyszyn, Vanoss, PANCHO GALAXIAS, FiLzO, -[SaDA]- NuLL, iRAGE, Deika, CONRECON, X dude Awesome, MarioDox, Spastik